Among potential targets for antiviral therapy that arise during certain viral infections are the virus-coded proteinases. These enzymes, essential for the synthesis of infectious virus, are required to process virus-specific precursor proteins involved in the maturation, assembly and replication of such pathogenic human viruses as adenovirus, poliovirus, encephalitis virus, hepatitis A and C viruses, rhinovirus, influenza virus, herpes virus, cytomegalovirus, and human immunodeficiency virus. Virus-coded proteinases are highly specific for their virus-coded substrates. If equally specific inhibitors can be developed and targeted to infected cells, they should interfere with virus replication and not with normal cellular metabolism. Our model system is the infection of human cells in culture by human adenovirus serotype 2 (Ad2). Our laboratory has shown: For maximal Ad2 proteinase activity in vitro, three components are required- the protein product of the adenovirus L3 23K gene, an 11 amino acid peptide (pVIc) that originates from the C-terminus of virion precursor protein pVI, and the viral DNA. The cofactors increase kcat 1,100-fold with pVIc and 15,800-fold with Ad2 DNA as well. There is a third cofactor, a cellular cofactor- actin. The three-dimensional crystal structure of the proteinase complexed with pVIc was solved at 1 .6 A resolution. The fold of the protein is unique; the proteinase represents a new class of cysteine proteinases. One objective in this grant proposal is to understand at the biochemical and structural levels how the activity of the adenovirus proteinase (AVP) is regulated: Why the enzyme is inactive initially and how the 3 cofactors regulate enzyme activity. The second objective is to use this biochemical and structural information to design proteinase inhibitors to act as antiviral agents. Structure-based drug design will be used to obtain proteinase inhibitors. Two inhibitors, one reversible and the other an irreversible inhibitor, have already been found. Drugs against three different sites on AVP will be used in a new form of clinical therapy that may prevent drug resistance from arising.